The wireless communications industry has experienced an explosive growth in the last decade. However, the available spectrum for wireless communications has not grown at the same rate. Increased cost of acquiring spectrum to accommodate users has resulted in an increased interest in spectrum efficient techniques that use multiple transmit and receive antennas instead of the conventional single transmit and receive antennas.
In conventional wireless communications, a single antenna is used at the source and the destination. In some cases, this gives rise to problems with multipath effects. When an electromagnetic field is met with obstructions such as hills, canyons, buildings, and utility wires, the wavefronts are scattered, and thus they take many paths to reach the destination. The late arrival of scattered portions of the signal causes problems such as fading, cut-out (cliff effect), and intermittent reception (picket fencing). In digital communications systems such as wireless internet, these problems can cause a reduction in data speed and an increase in the number of errors. The use of two or more antennas, along with the transmission of multiple signals (one for each antenna) at the source and the destination, can take advantage of the multipath wave propagation.
One of the most promising spectrum efficient techniques are multiple-input, multiple-output (MIMO) systems. MIMO is an antenna technology for wireless communications in which multiple antennas are used at both the source (transmitter) and the destination (receiver). The antennas at each end of the communications circuit are combined to minimize errors and optimize data speed. These systems exploit the spatial dimension to a larger extent than previous systems and have been shown to be capable of supporting very high data rates without increasing the bandwidth. MIMO is a reliable technique and has been put into practice in production of WLANS. MIMO is one of several forms of smart antenna technology, the others being Multiple Input Single Output (MISO) and Single Input Multiple Output (SIMO). MIMO technology has aroused interest because of its possible applications in digital television (DTV), WLANs, metropolitan area networks (MANs), and mobile communications.
What are needed, therefore, are methods and systems for more fully utilizing multiple transmit and/or multiple receive paths.